Diversity of seed plants and their systematics

entire female gametophyte becomes cellular (Fig. 17E). The peripheral cells are small, isodiametric and without
any food reserves, whereas the inner cells are large, irregular and packed with starch.
Archegonium
At the micropylar tip of the gametophyte, two to six archegonia differentiate. The archegonial initial (Fig.
18A;20F) divides to form a primary neck cell and a central cell. The former divides anticlinally, resulting in two
neck cells. (Fig. 20G) They divide once again, just before fertilization, to form four neck cells (Fig 18G; 20H).
The second wall is much thinner than the first one. By the time the nucleus of the central cell is ready to divide,
the neck cells become large and turgid and project into the archegonial chamber. The central cell continues to
grow for several months before its nucleus divides to form the ventral canal nucleus and egg nucleus (Fig. 18C)
where it soon disorganizes. Occasionally, however, it enlarges and simulates the egg nucleus. The maturation of
different archegonia in a gametophyte is synchronous. The egg cytoplasm shows a number of proteid vacuoles
which increase after fertilization. In C. rumphii they are rich in proteins (De Silva and Tambiah 1952). The
gametophytic region around the archegonia grows upwards and forms a depression in the middle, the
archegonial chamber. The archegonia occur singly, and each has a well-developed single layered jacket. There
is usually a thick wall with simple pits between the jacket cells and the central cell of the archegonium (Fig. 18
C,D). According to Swamy (1948), the pores become occluded by plug-like thickenings, and the jacket cells
become depleted of their contents. The thick cell wall forms the so-called egg-membrane during the postfertilization
stages. The egg membrane thickens as the egg matures. It retains its form and remains connected
with the suspensor of the embryo when the latter is fairly well advanced. There are prominent pits on the thick
wall. Extension of the egg cytoplasm come in contact with the cytoplasm of the jacket cells, through the
plasmodesmata present in the pit areas.
POLLINATION
The male cones mature when the young megasporophylls have emerged on the female plant, and the ovules are
at the free-nuclear gametophytic stage and ready for pollination. The single integument is drawn out into a short
micropylar tube, which secretes a sugary exudate (pollination drop) (Fig. 15) close to the nucellus of the ovule.
The wingless pollen grains usually float into the pollination drop, which retracts soon after. The pollen grains
show a germinal furrow, which closes in dry weather and is wide open in high humidity. After landing on the
pollination drop, the pollen grains imbibe water and other substances from the exudate, become heavy and sink
down the micropyle to reach the nucellus, where they germinate.
The male conses of Cycas growing in warm climate emit a strong odour (described earlier); several insects visit
them when the pollen ripens (see Chamberlain 1935, Pant 1973). Thus, these insects may also be carrriers of
pollen in cycads, Following pollination, the ovules enlarge and reach their full size. unpollinated ovules remain
small and eventually dry up.
Tang 1987, has reported inscet pollination in Zamia pumila and Norstog et at, 1986 reported role of beefless in
pollintion in Z furacea) Pant and Singh (1990) found that in Cycas, beetles, insects and even ants visited only
the male cones and continued to live there till the next generation. They did not find there beetles, etc.
pollinating the ovules in female plants. in Bowerta surrulata pollination is amphiptition (see Wilson 1993)
Tang (1987) reported widespread heat production in the male cones of 42 cycad species followed a circadian
rhythm. It is released every evening during cone elongation and pollen shedding stage. Concurrent to heat
production is the release of odours that helped in attracting insects for pollination.
After the pollen grains land in the pollen chamber (situated in the upper beak-shaped portion of the nucellus), a
group of cells lying below begin to degenerate and extend the lower limit of the chamber. This post-pollination
extension is the lower pollen chamber or intermediary chamber.
The intine of the pollen grain ruptures the exine at the distal end, grows into a tube in the region of the furrow,
and the tube nucleus migrates into it. The pollen tube grows laterally and intercellular into the tissue of the
nucellus, instead of towards the archegonia as in other gymnosperms. During its course through the nucellus
(Fig. 20E;21A), the tube often branches horizontally, derives nourishment as it digests its way, and accumulates
considerable amount of starch for the development of the male gametophyte. The pollen tube does not transport
the male gamete but establishes only a hausterial system.
(In Zamia furfuraea Choi & Friedman (1991) The pollen tube, while growing intercellularly in the nucellar
tissue, develops additional localized hausterial growth which penetrate adjoining nucellar cells. This
intracellular hausterial growth leads to the death of individual cells and also induce the degeneration of
neighbouring nuclellar tissue. The pollen tube, thus, act more as a hausterium than a sperm carrier which is its
normal function in other gymnosperms. In this respect, cycads, together with Ginkgo, can be said to have the
most primitive male gametophyte among the living gymnosperms.)
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